Solving hard to find "XYZ is not a constructor" JavaScript errors in Magento

It took me quite a few hours to find the cause of various mysterious errors in
Magento's compiled JavaScript files. On most pages these errors did not occur, only
on a specific product review page. Magento was told to leave some .js files away on
this url. So there was a difference between the compiled .js file on that review url
and the other urls. The thing which drived me nuts was: how can I have an error in
JavaScript code which just has less code than some other file which was even
bigger, but had no error?

Take a look at the following two snippets. The first one does not throw an error:

The reason is a silly missing semicolon after the definition of "Product.Config.prototype".
This is only a problem if the immediately following code starts with a parenthesis,
like in the second snippet.

In Magento versions up to the newest v1.9 you will find such prototype definitions without a
semicolon in the folder /js/varien/. Adding a semicolon at the end of
these saved me from getting even more grey hair than I already have.